The present invention relates to a display apparatus including a CRT (cathode-ray tube) such as a television set or a display monitor.
Generally, in a display apparatus including a CRT, electrons emitted from a cathode of an electron gun disposed at a neck of the CRT are focused by an electrostatic lens to converge at a fluorescent screen on the anode side. Such an electrostatic lens disposed within the electron gun is comprised of focusing electrode (generally referred to as a G3 electrode) which is applied with a focusing dc voltage.
The distance between the focusing electrode and the fluorescence screen, that is, the distance which electrons emitted form the cathode travel, varies with the points in the fluorescent screen into which they run. Accordingly, to obtain a favorable focusing characteristic throughout the screen of the CRT, it is necessary to vary the focusing voltage applied to the focusing electrode in synchronization with deflection of the electron beam. For this reason, a dynamic focusing circuit has been used to modulate the focusing voltage by use of a signal having a waveform varying in synchronization with horizontal and vertical deflections of the electron beam.
FIG. 4 shows a structure of a conventional display apparatus having such a dynamic focusing circuit. This apparatus includes a video amplifier 1, a CRT 20, a high-voltage generating circuit 10, resistor-dividing circuit 30, and a dynamic focusing circuit 40.
A conventional CRT electron gun includes a G1 electrode, a G2 electrode, and a G3 electrode disposed in that order from a cathode for drawing electrons from the cathode and focusing them. On the other hand, as shown in FIG. 4, this display apparatus uses a CRT 20 having an electron gun in which, in addition to a G1 electrode 21, a G2 electrode 22, and a G3 electrode 23, 24, a Gm electrode 26 for controlling flow of the electron beam is disposed between the G2 electrode and the G3 electrode. This CRT 20, which is referred to as a xe2x80x9cHi-Gm tubexe2x80x9d, is disclosed in Japanese Unexamined Patent Publication No. 224618/99.
In this figure, video signals of R, G, and B from the video amplifier 1 are supplied to a cathode 25. The high-voltage generating circuit 10 which includes a flyback transformer FBT rectifies a high pulse voltage generated in the secondary coil of the flyback transformer to produce an anode high voltage Eh to be applied to an anode 28 of the CRT 20. This anode high voltage Eh is divided by the resistor-dividing circuit 30 which includes variable resistors VR1, VR2, and resistors R30, R31, so that focusing dc voltages Ef1 and Ef2 to be applied to the G3 electrode 23 serving as a static focusing electrode and the G3 electrode 24 serving as a dynamic focusing electrode respectively are output from the variable resistors VR1 and VR2 respectively.
The dynamic focusing circuit 40 is supplied with, from outside, a horizontal parabola-form voltage whose amplitude varies along a parabola in synchronization with horizontal deflection of an electron beam and a vertical parabola-form voltage whose amplitude varies along a parabola in synchronization with vertical deflection of the electron beam. The dynamic focusing circuit 40 mixes the horizontal and vertical parabola-form voltages, and the resultant composite voltage is superimposed on the focusing voltage output from the variable resistor VR2 through a coupling capacitor CO to produce a dynamic focusing voltage. This dynamic focusing voltage is applied to the G3 electrode 24 as a focusing voltage which has been compensated for the variation of the distance between the focusing electrode and the fluorescence screen.
However, when the brightness of the screen is high, or when the intensity of the anode current which is substantially equal to the electron beam current is high, the focusing characteristic deteriorates even if such a dynamic focusing circuit is used. That is because, as shown in FIG. 5, a CRT has the characteristic that Ef/Eh (%), which is preadjusted to the best focusing point that exists within the range of 20% to 50% normally, decreases as Ip increases where Ef is an optimum static or dynamic focusing voltage and Ip is an anode current, and accordingly, as the brightness increases, that is, as the anode current increases, the fixed focusing voltage produced by dividing the anode high voltage Eh under the fixed division ratio distances from the optimum focusing voltage.
In a normal-brightness state (when the anode current is below 0.8 mA for example), a shift from the optimum value is relatively small and the focus-deterioration is negligible. However, in a high-brightness state (when the anode current is from 0.8 mA to 2.4 mA for example), the shift is as much as 2% to 3% (a hatched portion in FIG. 5), and therefore the focus-deterioration is not negligible where image quality is degraded conspicuously.
The above-described problem becomes more acute in a display apparatus using the Hi-Gm tube. That is because, since the amplitude of a cathode voltage required to generate the same anode current in the Hi-Gm tube is less than half the amplitude required of a conventional CRT, and therefore the anode current (electron beam) can be more than twice the anode current generated in the conventional CRT for the same amplitude of the cathode voltage, high contrast can be obtained more easily, but the situation of a large anode current flowing (or high-brightness state) occurs more frequently for that.
The present invention has been made to solve the above-described problem with an object to provide a display apparatus which does not exhibit the deterioration in the focusing characteristic in a high-brightness state. This object is achieved by a display apparatus including:
a CRT provided with an electron gun having a focusing electrode which is applied with a dynamic focusing voltage for focusing electrons drawn from a cathode of the CRT;
a dynamic focusing circuit for producing the dynamic focusing voltage by superimposing, on a dc voltage supplied from outside, an ac voltage which has a waveform varying in synchronization with horizontal and vertical deflections of an electron beam flowing to a fluorescent screen on an anode side of the CRT; and
a compensator for lowering the dc voltage supplied to the dynamic focusing circuit by a predetermined value while a brightness signal indicative of brightness of a screen of the CRT exceeds a threshold value.
The electron gun may have the cathode, and a G1 electrode, a G2 electrode and a G3 electrode disposed in that order for drawing electrons from the cathode, the G3 electrode serving as the focusing electrode, the electron gun further having a Gm electrode between the G2 and G3 electrodes for controlling flow of the electron beam.
The display apparatus may further include a voltage divider for dividing a high voltage applied to an anode of the CRT to produce the dc voltage, the voltage divider including a variable resistor and a dividing resistor connected in series across the anode and a ground.
The compensator may include an operational amplifier for outputting a difference between a voltage across a brightness-detection resistor through which an anode current flows and a reference voltage, and an impedance control circuit including a resistor having a predetermined impedance and a switching circuit for connecting or disconnecting the resistor in parallel with the dividing resistor depending on polarity of the difference output by the operational amplifier.